A conventional hydraulic clutch actuating system for motor vehicles has a master cylinder connected with an equalizing reservoir filled with hydraulic fluid. The master cylinder has a cylinder housing with a housing bore in which a piston is received to be longitudinally displaceable. The master cylinder piston bounds a pressure chamber in the housing bore of the cylinder housing and can be mechanically loaded with an actuating force by a clutch pedal or an electric-motor drive. The pressure chamber of the master cylinder is hydraulically connected with a pressure chamber of a slave cylinder by way of a pressure line so that the pressure generated in the pressure chamber of the master cylinder by depressing the clutch pedal or electric-motor displacement of the master cylinder piston is transmissible to the pressure chamber of the slave cylinder by way of a fluid column in the pressure line. The slave cylinder similarly includes a cylinder housing with a housing bore in which a piston bounding the pressure chamber of the slave cylinder and thus loadable with the pressure generated in the master cylinder is received to be longitudinally displaceable. Associated with the slave cylinder piston is a piston rod engaging a clutch lever which in turn is in operative connection with the release bearing of the clutch. As a result, the release bearing of the clutch can be loaded with an actuating force via the slave cylinder piston so as to separate the clutch pressure plate from the clutch drive disc by way of a release mechanism and thus the motor from the transmission of the motor vehicle.
An actuating cylinder, particularly a slave cylinder for a hydraulic clutch actuating system for motor vehicles, which comprises a cylinder housing in which a pressure chamber is formed, is known from document U.S. Pat. No. 5,836,235 The pressure chamber is variably bounded on one side by a piston of a piston subassembly, which is received longitudinally displaceably in the cylinder housing. A piston rod is rigidly connected with the piston such that the cost associated with the pivotable coupling of the piston rod to the piston as widely used in the prior art is avoided. The piston has an outer contour permitting slight tilting of the piston subassembly in the cylinder housing. In addition, the piston has an encircling groove, which extends in axial direction and is open towards the pressure chamber, for reception of an annular seal, i.e. groove ring centered and retained on an extension of the piston at the pressure chamber side by a retaining ring, i.e., spring plate, which is engaged by a helical compression spring received in the pressure chamber. The seal has an encircling sealing lip that slidably and sealably bears against a sliding surface of the cylinder housing at the inner circumference.
The encircling groove extends in axial direction and is open towards the pressure chamber of the piston. A flank of the piston thereof facing the cylinder wall ensures that the seal is not excessively deformed even when pivoting of the piston rod or the piston relative to the cylinder housing takes place, so that sufficient fluid tightness of the actuating cylinder shall be provided.
This construction may be applicable to the installed state of the actuating cylinder where the piston rod is guided by its end remote, i.e., opposite from the piston or is mounted at a coupling lever and also the piston is mostly disposed in a stroke setting in which the piston rod in the case of tilting of the piston could if necessary come into contact with the cylinder housing. However, it can be problematic if the actuating cylinder is demounted, for example, in the course of repairs to the motor vehicle, so that the piston rod is no longer guided or held. If the piston as a consequence of its spring loading then travels into a stroke setting near the open end of the cylinder housing, where even the cylinder housing cannot sufficiently limit pivotation of the piston rod, then pivot angles of, for example, equal to or greater than 7° between the piston rod and the center axis of the cylinder housing can easily arise. However, in the case of such comparatively large pivot angles it is possible for resilient deformation of the seal to occur to such an extent that leakages arise and wet the actuating cylinder. It can then easily happen that the actuating cylinder is incorrectly regarded as defective and exchanged, although in remounted state it would readily fulfill its function.
What is desired is to create a hydraulic cylinder for a hydraulic actuating system for motor vehicles, which always particularly even in the unmounted state has sufficient fluid tightness.